SIMPLE Working Group C. Holmberg
Internet-Draft Ericsson
Intended status: Standards Track S. Blau
Expires: June 12, 2011 Ericsson AB
December 9, 2010
Session Matching Update for the Message Session Relay Protocol (MSRP)draft-ietf-simple-msrp-sessmatch-10.txt
Abstract
This document defines an extension, sessmatch, for the Message
Session Relay Protocol (MSRP) session matching procedure of MSRP
entities. The extension extends the applicability of MSRP
communication to network scenarios where Application Layer Gateway
(ALG) functions modify the Session Description Protocol (SDP) MSRP
address information.
Status of this Memo
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This Internet-Draft will expire on June 12, 2011.
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Internet-Draft MRSP December 20101. Introduction
The Message Session Relay Protocol (MSRP) [RFC4975] is designed to
use MSRP relays [RFC4976] as a means for Network Address Translation
(NAT) traversal and policy enforcement.
However, many Session Initiation Protocol (SIP) [RFC3261] networks,
in which MSRP usage is emerging, also contain SIP Application Layer
Gateways (ALGs), which anchor and controls media, perform tasks such
as NAT traversal, performance monitoring, lawful intercept, address
domain bridging, interconnect Service Layer Agreement (SLA) policy
enforcement, etc. An example is the Interconnect Border Control
Function (IBCF) [3GPP.23.228] defined by the 3rd Generation
Partnership Project (3GPP), which controls a media relay that handles
all types of SIP session media (voice, video, MSRP, etc).
MSRP, as defined in RFC 4975 [RFC4975] and RFC 4976 [RFC4976], does
not work when an MSRP entities communicate with such ALGs, unless the
ALGs implement MSRP Back-To-Back User Agent (B2BUA) functionality.
The reason is that entities use the MSRP URI comparison [RFC4975]
procedure in order to match an MSRP message to an MSRP session. That
requires consistency between the address information in the MSRP
messages and the address information carried in the SDP a=path
attribute. The matching will fail if ALGs modify the address
information of the SDP a=path attribute, but do not implement MSRP
B2BUA functionality and perform the corresponding modification in the
associated MSRP messages. However, few ALGs implement MSRP B2BUA
functionality, due to complexity and poor scalability.
This specification defines an MSRP extension, sessmatch, that allows
MSRP entities to communicate with ALGs that do not implement MSRP
B2BUA functionality. MSRP entities that support the sessmatch use a
different mechanism for matching an MSRP message with an MSRP
session, called session matching. Instead of using the MSRP URI
comparison procedure defined in RFC 4975, only the MSRP session-id
part is used for the session matching by entities that support the
sessmatch extension.
In case TLS with name based authentication is used, ALGs need to
implement MSRP B2BUA functionality also when communicating with MSRP
entities that support the sessmatch extension, in order to prevent
the MSRP communication from failing due to a certificate mismatch.
The sessmatch extension is backward compatible. In the absence of
ALGs, MSRP entities that do not implement the sessmatch extension can
interoperate with entities that do implement it. The reason is that
the matching of an MSRP message to an ongoing session will not fail.
MSRP entities that do not implement the sessmatch extension, and
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Internet-Draft MRSP December 2010
communicate with ALGs that do not implement MSRP B2BUA functionality,
can normally not establish MSRP sessions, since the session matching
will fail in case the address information of the SDP a=path attribute
has been modified by the ALGs.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
In this specification the terminology "fingerprint based TLS
authentication" and "name based TLS authentication" are used to refer
to the two cases where:
1. An endpoint use a self-signed TLS certificate and sends a
certificate fingerprint in SDP (fingerprint based TLS
authentication).
2. An endpoint use a certificate from a well known certificate
authority and the other endpoint matches the hostname in the received
TLS communication SubjectAltName parameter towards the hostname
received in the MSRP URI in SDP (name based TLS authentication).
3. Applicability statement
This document defines an MSRP extension, sessmatch. Support of the
extension is optional. MSRP entities can implement the extension in
order to allow MSRP communication in networks where ALGs that might
modify the address information of the SDP a=path attribute, but do
not implement MSRP B2BUA functionality, are present.
4. Sessmatch mechanism4.1. General
This section defines how an MSRP entity that supports the sessmatch
extension performs session matching, i.e. matches an incoming MSRP
message to an MSRP session.
4.2. Session matching
The difference between the session matching mechanism in RFC 4975,
and the one defined in this specification for the sessmatch
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Internet-Draft MRSP December 2010
extension, is that while the mechanism in RFC 4975 uses the MSRP URI
comparison rules for session matching, the sessmatch extension only
uses the session-id part of the MSRP URI.
When an MSRP entity that receives the first MSRP request for an MSRP
session, the To-Path header field of the request should contain a URI
with a session-id part that was provided in the SDP associated with
the MSRP session. The entity that accepted the connection looks up
the session-id part of the MSRP URI in the received requests, in
order to determine which session it matches. The session-id part is
compared as case sensitive. If a match exists, the entity MUST
assume that the host that formed the connection is the host to which
this URI was given. If no match exists, the entity MUST reject the
request with a 481 response. The entity MUST also check to make sure
the session is not already in use on another connection. If the
session is already in use, it MUST reject the request with a 506
response.
NOTE: As the sessmatch extension, in a peer to peer session, is
backward compatible with the RFC 4975 mechanism, the SIMPLE WG did
not see a need to define a SIP option-tag associated with the
sessmatch extension. In case the session path contains an
intermediary that modifies the SDP MSRP routing information, MSRP
session establishments that contain RFC 4975 entities will fail.
However, that is the case even without the sessmatch extension.
Also, an intermediary will normally make a decision whether to insert
itself in the session path when it receives the SDP offer. However,
it will not be aware about whether the MSRP endpoint acting as SDP
answerer supports the sessmatch extension until it receives the SDP
answer.
5. Security Considerations5.1. MSRP URI as shared secret
An MSRP entity that does not support the sessmatch extension uses the
complete MSRP URI (scheme, authority, transport, session-id) as a
shared secret in order to determine that an incoming transport
connection originates from the intended peer device. The shared
secret needs to be hard to guess, but in reality only the session-id
part with it's minimum 80 bit of randomness is hard to guess. Using
only the MSRP URI session-id part as shared secret is therefore
roughly as good as using the complete URI.
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Internet-Draft MRSP December 20105.2. Uniqueness of the session-id
The value of the MSRP URI session-id part only needs to be unique
within the scope of the MSRP entity that created it, so in order to
make the session-id unique there is no need to scope its namespace by
the MSRP URI authority part.
5.3. Man in the middle
A man-in-the-middle (MiTM) that wants to insert itself in the MSRP
communication path, in order to modify unprotected MSRP messages,
needs to implement MSRP B2BUA functionality. If the MiTM
communicates with MSRP entities that support the sessmatch extension,
it does not need to modify the To-Path and From-Path header fields in
the MSRP messages, which is the case if it communicates with an MSRP
entity that do not support the extension. In both cases, however,
the MiTM needs to terminate the TCP/TLS connection used for the MSRP
communication.
The sessmatch extension makes it easier for a MiTM to monitor and
record unprotected MSRP communication, as it allows the MiTM to
anchor the MSRP communication even if it does not implement MSRP
B2BUA functionality.
The sessmatch extension does not make it easier for a MiTM to insert
itself in the SIP/SDP signaling path, neither does it make it easier
for a MiTM to forward MSRP messages towards malicious MSRP entities
(as it does not require the MiTM to anchor the MSRP communication).
5.4. TLS
This specification does not in any way modify TLS security procedures
as such. The sessmatch extension allows the usage of ALGs that do
not implement MSRP B2BUA functionality in MSRP communications, unless
TLS with name based authentication is used, and unless MSRP relays
are used. In such cases ALGs need to implement MSRP B2BUA
functionality, to prevent the MSRP communication from failing. That
applies to MSRP in general, and is not specific to the extension
defined in this specification.
In case TLS with fingerprint based authentication is used, the
sessmatch extension allows usage of ALGs that modify the address
information of the SDP a=path attribute, but no not implement MSRP
B2BUA functionality, to communicate with MSRP entities. In order to
use fingerprint authentication, the SDP that carries the fingerprint
information needs to be integrity protected. In case an ALG needs to
be able to modify SDP information, however, it will not be possible
to provide full end-to-end SDP integrity protection of the SDP.
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